Optical amplification apparatus utilizing Raman amplification and controlling method thereof

ABSTRACT

The present invention aims at providing an optical amplification apparatus for improving noise characteristics by controlling an amplification operation by assuming a noise figure of the overall apparatus and by taking influences of noise light due to Raman amplification into consideration, and a controlling method of the optical amplification apparatus. The optical amplification apparatus according to the present invention comprises first optical amplifying means for Raman amplifying signal light by supplying excitation light to a Raman amplification medium, second amplifying means for amplifying signal light output from the first optical amplifying means, target value setting means for setting a target value that minimizes the noise figure of the overall apparatus as to input light power of the second optical amplifying means, and excitation light controlling means for controlling an excitation light supply condition of the first optical amplifying means in accordance with the target value set by the target value setting means. Consequently, the noise characteristics of the overall apparatus can be optimized by the control inside the own apparatus and excellent noise characteristics can be accomplished.

This application is a continuation of PCT/JP00/05885 filed on Aug. 30,2000.

TECHNICAL FIELD

The present invention relates to an optical amplification apparatus foramplifying signal light by utilizing Raman amplification and acontrolling method thereof. More particularly, present invention relatesto an optical amplification apparatus for achieving the improvement ofnoise characteristics of the overall optical amplification apparatus bytaking influences of noise light generated by Raman amplification intoconsideration, and a controlling method thereof.

RELATED ART

Demands for information have been increased drastically in recent yearswith the progress of Internet technologies. A greater capacity and theformation of more flexible networks have been required in a trunk typeoptical transmission system in which an information capacity isintegrated. A WDM optical transmission system for transmittingwavelength division multiplexed (WDM) signal light obtained bymultiplexing a plurality of optical signals having different wavelengthsis one of the most effective means that can cope with such a systemdemand. In a conventional WDM optical transmission system, an opticalfiber amplifier using an optical fiber doped with a rare earth elementsuch as erbium (Er) is utilized, as an optical repeater. By utilizingbroadband characteristics of this optical fiber amplifier, one opticalfiber can realizes WDM optical transmission for repeating andtransmitting optical signals of a plurality of wavelengths.

To further increase the capacity and to extend the distance and therepeating interval in the WDM optical transmission system as describedabove, means for compensating for the degradation of S/N in thetransmission system becomes necessary. For this purpose, it is effectiveto use means for supplying excitation light to a transmission path, toperform distributed Raman amplification of the transmission p path byutilizing an amplification operation using the effect of stimulatedRaman scattering, so that the repeating loss is equivalently reduced, inaddition to an existing optical amplification repeating transmissionsystem.

FIG. 8 is a structural diagram showing the outline of a WDM opticaltransmission system using distributed Raman amplification, which hasbeen proposed heretofore.

In the WDM optical transmission system in FIG. 8, a transmission path 3connects a transmission station (Tx) 1 and a reception station (Rx) 2,and a plurality of optical repeaters 4 are arranged on the transmissionpath 3 with predetermined intervals so that WDM signal light istransmitted and repeated from the transmission station 1 to thereception station 2. Each optical repeater 4 includes an opticalamplification apparatus constituted by combining a DRA (DistributedRaman Amplifier) with an EDFA (Erbium-Doped Fiber Amplifier). In thisDRA, excitation light for Raman amplification (hereinafter called “Ramanexcitation light”) generated in an excitation light source is suppliedthrough an optical coupler to the transmission path 3 connected to thetransmission station side, and the WDM signal light propagated throughthe transmission path 3 is subjected to distributed Raman amplification.The WDM signal light subjected to the distributed Raman amplification isinput to the EDFA, to be amplified to a necessary level, and is againoutput to the transmission path 3. With such a WDM optical transmissionsystem, since the loss in the transmission path 3 in each repeatingsegment is decreased due to distributed Raman amplification,transmission characteristics of the WDM signal light can be improved.

Noise characteristics of the optical amplification apparatus constitutedby combining the DRA with the EDFA and used for such a WDM opticaltransmission system are affected not only by the noise figure (NF) ofthe EDFA but also by noise light generated by the Raman amplification.The noise light resulting from Raman amplification is generated alsowhen only Raman excitation light is incident to an amplification mediumunder a state where signal light is not input, and is generally called“Raman scattering light due to pumping light”. Here, noise lightgenerated in the DRA is called “Amplified Spontaneous Raman Scattering(ASS) light” in contrast with Amplified Spontaneous Emission (ASE) lightgenerated in the EDFA.

To improve the noise characteristics of the optical amplificationapparatus and to further improve the transmission characteristics, it isnecessary to reduce the noise figure of the overall opticalamplification apparatus by taking the influences of ASS light intoconsideration. To improve the noise characteristics of the opticalamplification apparatus, technologies for reducing independently thenoise figure of the EDFA have been studied in the past, but specificconsiderations taking the influences of ASS into account have not beenmade.

The present invention has been made in view of the problems describedabove, and it is an object of the present invention to provide anoptical amplification apparatus for achieving the improvement of noisecharacteristics by controlling an amplification operation by assuming anoise figure of an overall optical amplification apparatus while takinginfluences of noise light resulting from Raman amplification intoaccount, and a controlling method of such an optical amplificationapparatus.

DISCLOSURE OF THE INVENTION

To accomplish the object described above, an optical amplificationapparatus utilizing Raman amplification according to the presentinvention comprises: first optical amplifying means for Raman amplifyingsignal light propagated through a Raman amplification medium bysupplying excitation light to the Raman amplification medium; and secondoptical amplifying means for amplifying the signal light output from thefirst optical amplifying means, wherein the optical amplificationapparatus further comprises: target value setting means for setting atarget value for minimizing a noise figure of the overall opticalamplification apparatus as to input light power of said secondamplifying means; and excitation light controlling means for controllingan excitation light supply condition of the first optical amplifyingmeans in accordance with the target value set by the target valuesetting means.

According to this construction, input light power of the second opticalamplifying means to which Raman amplified signal light is input, istaken into specific consideration, and its target value is set by thetarget value setting means.

The input light power target value of the second optical amplifyingmeans minimizes the noise figure of the overall optical amplificationapparatus constituted by combining the first and second amplifyingmeans. As the excitation light supply condition of the first opticalamplifying means is adjusted by the excitation light controlling meansin accordance with the set target value, actual input light power of thesecond optical amplifying means is so controlled as to coincide with thetarget value. Consequently, the noise characteristics of the overalloptical amplification apparatus, that takes the influences of noiselight due to Raman amplification into consideration, can be optimized bythe control inside its own apparatus, and an optical amplificationapparatus having excellent noise characteristics can be realized.

As one aspect of the optical amplification apparatus described above ,the target value setting means may include an excitation light powerdetecting section for detecting excitation light power supplied to theRaman amplification medium, and a computing section for computing noiselight power by the first optical amplifying means in accordance with adetection result of the excitation light power detecting section, andsetting an input light power target value of the second opticalamplifying means for minimizing the noise figure of the overall opticalamplification apparatus on the basis of the computed noise light powerand on the basis of noise characteristics of the second opticalamplifying means.

According to this aspect, in the computing section, the input lightpower target value of the second optical amplifying means is obtained onthe basis of noise light power due to Raman amplification computed inaccordance with the power of Raman amplification excitation lightdetected by the excitation light power detecting section, and on thebasis of the noise characteristics of the second optical amplifyingmeans. The excitation light controlling means executes its controllingoperation in accordance with this target value.

As another aspect of the optical amplification apparatus describedabove, the target value setting means may set a maximum value of inputdynamic range of the second optical amplifying means to the input lightpower target value of the second optical amplifying means. Further, inthis case, it is preferred that when excitation light power of the firstoptical amplifying means reaches a maximum value before the input lightpower of the second optical amplifying means reaches the maximum valueof the input dynamic range, the target value setting means sets theinput light power target value of the second optical amplifying means soas to correspond to the maximum value of excitation light power of thefirst optical amplifying means.

According to this aspect, the input light power target value of thesecond amplifying means is set in the target setting means to themaximum value of the input dynamic range of the second opticalamplifying means, and the excitation light controlling means executesits controlling operation in accordance with the target value. At thistime, if the excitation light power for Raman amplification reaches themaximum output before the input light power of the second opticalamplifying means reaches the maximum value of the input dynamic range,the input light power corresponding to the excitation light power atthat point is set as the target value.

The optical amplification apparatus described above may further includeinput light power detecting means for detecting the input light power ofthe second optical amplifying means, and the excitation lightcontrolling means may control the excitation light supply condition ofthe first optical amplifying means so that a detection result of theinput light power detecting means coincides with the target value set bythe target value setting means. This construction makes it possible toperform a feedback control that keeps the input light power of thesecond optical amplifying means to be constant at the target value.

As a specific construction of the optical amplification apparatusdescribed above, when the second optical amplifying means includes aplurality of optical amplifying sections connected in parallel with oneanother, the excitation light controlling means may set an input lightpower target value corresponding to each of the optical amplifyingsections. A specific construction of the second optical amplifying meansmay include an optical fiber amplifier using a fiber doped with a rareearth element.

With a method of controlling an optical amplification apparatusutilizing Raman amplification according to the present invention, in theoptical amplification apparatus comprising: first optical amplifyingmeans for Raman amplifying signal light propagated through a Ramanamplification medium by supplying excitation light to the Ramanamplification medium; and second optical amplifying means for amplifyingthe signal light output from the first optical amplifying means, atarget value for minimizing a noise figure of the overall opticalamplification apparatus as to input light power of the second amplifyingmeans is set and an excitation light supply condition of the firstoptical amplifying means is controlled in accordance with the targetvalue.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a construction of an opticalamplification apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a diagram showing a relation between total power of noisecomponents (ASS light) generated by Raman amplification and power ofRaman excitation light;

FIG. 3 is a conceptual view for explaining an operation of a computingsection in the first embodiment of the present invention;

FIG. 4 is a view of a virtual construction for explaining the operationof the computing section in the first embodiment of the presentinvention;

FIG. 5 is a diagram showing dependence of a noise figure of EDFA oninput light power in the first embodiment of the present invention;

FIG. 6 is a block diagram showing a construction of an opticalamplification apparatus according to a second embodiment of the presentinvention;

FIG. 7 is a diagram for explaining the calculation of an SRS tilt andthe initial value setting of a Raman excitation light power ratio in thesecond embodiment of the present invention; and

FIG. 8 is a structural diagram showing the outline of a known WDMoptical transmission system that utilizes distributed Ramanamplification.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of an optical amplification apparatus utilizing Ramanamplification according to the present invention will be explainedhereinafter with reference to the accompanying drawings.

FIG. 1 is a block diagram showing the construction of an opticalamplification apparatus according to a first embodiment of the presentinvention.

Referring to FIG. 1, the optical amplification apparatus includes, forexample, an excitation power source 10 and an optical coupler 12 asfirst optical amplifying means, an optical coupler 11 and a monitor 13as an excitation light power detecting section, a computing section 14,a controlling section 15 as excitation light controlling means, EDFA 20as second optical amplifying means, and an optical coupler 31 and amonitor 32 as input light power detecting means.

The excitation light source 10 generates excitation light for Ramanamplification (Raman excitation light) having a wavelength set inadvance so as to correspond to a wavelength band of a WDM signal lightto be transmitted. This Raman excitation light is supplied to atransmission path 3 through the optical couplers 11 and 12. Thetransmission path 3 is the same as the transmission path in the WDMoptical transmission system shown in FIG. 8. The optical coupler 11branches a part of Raman excitation light output from the excitationlight source 10 and transmits it to the monitor 13. The optical coupler12 supplies the Raman excitation light having passed through the opticalcoupler 11 from a signal light input terminal to the transmission path 3and passes therethrough the WDM signal light from the transmission path3 to transmit to the EDFA side 20. Here, the Raman excitation light ispropagated in an opposite direction to the WDM signal light, and thetransmission path 3 connected to the signal light input terminal servesas a Raman amplification medium. In this way a distributed Ramanamplifier (DRA) is constituted, in which the WDM signal light propagatedthrough the transmission path 3 is Raman amplified.

The monitor 13 monitors power of Raman excitation light output from theexcitation light source 10 on the basis of branched light of the opticalcoupler 11, and outputs a monitoring result to the computing section 14.The computing section 14 computes total power of amplified spontaneousRaman scattering light (ASS light) as a noise component due to Ramanamplification, sets a target value of input light power to EDFA 20, thatminimizes a noise figure (NF) of the overall apparatus, and outputs thistarget value to the controlling section 15. Incidentally, a specificmethod of setting the target value in the computing section 14 will beexplained later. The controlling section 15 adjusts a driving conditionof the excitation light source 10 in accordance with the target valueset by the computing section 14 and the monitoring result of the monitor32, to control such as the power of Raman excitation light supplied tothe transmission path 3.

The EDFA 20 is an EDFA having a general construction that amplifies aWDM signal light having passed through the optical couplers 12 and 31 toa required level and then outputs this amplified light. This EDFA 20 hascharacteristics such that its noise figure is changed in accordance withinput light power. Information about the noise characteristics of thisEDFA 20 is assumed to be stored in advance in the computing section 14or to be transferred at an appropriate timing. Incidentally, the secondoptical amplifying means of the present invention is not limited to theEDFA, but may be an optical fiber amplifier doped with a rare earthelement other than erbium or a Raman amplifier.

The optical coupler 31 branches a part of WDM signal light input to theEDFA 20 described above and transmits branched light to the monitor 32.The monitor 32 monitors input light power of the EDFA 20 on the basis ofthe branched light from the optical coupler 31 and outputs itsmonitoring result to the controlling section 15.

Next, the operation of the first embodiment will be explained.

First, a specific explanation will be made on the setting process of thetarget value executed in the computing section 14 of the present opticalamplification apparatus.

In the computing section 14, total power of ASS light is computed on thebasis of the Raman excitation light power as described above. It hasbeen confirmed experimentally that total power of ASS light (noisecomponent) generated due to Raman amplification is changed with respectto power of Raman excitation light in accordance with the relation shownin FIG. 2, for example. When this relation is expressed by a numericformula by using true values, total power Ass [mW] of ASS light can beexpressed by the following equation (1): $\begin{matrix}{{Ass} = {{m_{1} \cdot 10^{\frac{{a_{11} \cdot {Pu}_{1}} + a_{10}}{10}}} + {m_{2} \cdot 10^{\frac{{a_{21} \cdot {Pu}_{2}} + a_{20}}{10}}} + \cdots + {m_{i} \cdot 10^{\frac{{a_{i1} \cdot {Pu}_{i}} + a_{i0}}{10}}}}} & (1)\end{matrix}$

where Pu₁ to Pu_(i) represent Raman excitation power [mW] generated ineach excitation light source when i numbers of excitation light sourcesfor Raman amplification, having mutually different wavelengths, aredisposed (i=1 in this embodiment), m₁ to m_(i) represent a weightingconstant corresponding to each excitation light source, and a₁₁, a₁₀ toa_(i1), a_(i0) represent constants (calculation coefficients) when therelation shown in FIG. 2 is approximated by a linear function. Althoughthe relation between the total power of ASS light and the power of Ramanexcitation light are herein approximated by the linear function,approximation can be made by functions of the second and higher degrees,to improve accuracy.

When the total power Ass of ASS light is computed by using the Ramanexcitation power measured by the monitor 13 in accordance with therelational formula (1), next, the input light power target value of theEDFA 20, that minimizes the noise figure of the overall opticalamplification apparatus constituted by combining the DRA and the EDFA,is obtained.

When the noise figure of the overall optical amplification apparatus isconsidered, it is possible to define the noise figure of the DRA in thefollowing way and then to calculate the noise figure of the overallapparatus on the basis of this DRA noise figure and the noisecharacteristics of the EDFA that are stored in advance.

It can be considered that in the optical amplification apparatusconstituted by combining the DRA and the EDFA, pumping of Ramanexcitation light output from the excitation light source 10 changes theloss of transmission path 3 from L to L_(NEW) and the input light levelof the EDFA 20 from Pi_(OLD) to Pi_(NEW) as shown in the conceptual viewof FIG. 3. If a gain of the DRA is Gain at this time, Gain can beexpressed as Gain=L/L_(NEW) or Gain=Pi_(NEW)/Pi_(OLD). Therefore, anamplifier block is herein assumed, that has a DRA having the Raman gainGain described above and a noise figure NF_(DRA) resulting fromgeneration of ASS light. An optical amplification apparatus having avirtual construction shown in FIG. 4 is assumed, too. A noise figureNF_(DRA+EDFA) of the overall apparatus is thus derived.

Generally, a noise figure NF_(DRA(OFF)) of the DRA when Raman excitationlight is OFF can be expressed by the following equation (2), and a noisefigure NF_(DRA(ON)) of the DRA when Raman excitation light is ON can beexpressed likewise by the following equation (3):

NF_(DRA(OFF))=L  (2)

NF_(DRA(ON))={P_(ASS)/(h·ν·Δf)+1}·L_(NEW)  (3)

where L represents the loss of transmission path when excitation lightis OFF, and L_(NEW) represents the loss of transmission path whenexcitation light is ON. Symbol h represents the Planck's constant, ν isa wavelength and Δf is a filter band (for example, 10 GHz). P_(ASS) is avalue obtained by converting the total power Ass of ASS light calculatedby the equation (1), to resolution (for example, 10 GHz) correspondingto a noise figure NF_(EDFA) of the EDFA to be used for subsequentcomputation, in the unit of dBm.

Here, when a degradation amount of the noise figure due to pumping ofthe DRA is taken into consideration, a virtual noise figure NF_(DRA) ofthe DRA can be defined. This NF_(DRA) can be expressed by the followingequation (4) from the relation of the equations (2) and (3) given above:$\begin{matrix}\begin{matrix}{{NF}_{DRA} = \quad {{NF}_{{DRA}{({ON})}}/{NF}_{{DRA}{({OFF})}}}} \\{= \quad \frac{{P_{ASS}/\left( {{h \cdot v \cdot \Delta}\quad f} \right)} + 1}{Gain}}\end{matrix} & (4)\end{matrix}$

Next, the noise figure of the overall apparatus constituted by combiningthe DRA and the EDFA is considered. A noise figure NF_(DRA+EDFA(OFF)) ofthe overall apparatus when Raman excitation light is OFF can beexpressed by the following equation (5), and a noise figureNF_(DRA+EDFA(ON)) of the overall apparatus when Raman excitation lightis ON can be expressed by the following equation (6): $\begin{matrix}{{NF}_{{DRA} + {{EDFA}{({OFF})}}} = {L \cdot {{NF}_{EDFA}\left( {Pi}_{OLD} \right)}}} & (5) \\{{NF}_{{DRA} + {{EDFA}{({ON})}}} = {L \cdot \left( {{NF}_{DRA} + \frac{{{NF}_{EDFA}\left( {Pi}_{NEW} \right)} - 1}{Gain}} \right)}} & (6)\end{matrix}$

where NF_(EDFA)(Pi_(OLD)) represents the noise figure of the EDFA atinput light power Pi_(OLD) when excitation light is OFF, andNF_(EDFA)(Pi_(NEW)) represents the noise figure of the EDFA at inputlight power Pi_(NEW) when excitation light is ON.

Here, a virtual noise figure NF_(DRA+EDFA) of the overall apparatus canbe defined in the same way as when the virtual noise figure NF_(DRA) ofthe DRA is considered. This NF_(DRA+EDFA) can be expressed by thefollowing equation (7) from the relation of the equations (4) to (6).$\begin{matrix}\begin{matrix}{{NF}_{{DRA} + {EDFA}} = \quad {{NF}_{{DRA} + {{EDFA}{({ON})}}}/{NF}_{{DRA} + {{EDFA}{({OFF})}}}}} \\{= \quad \frac{{P_{ASS}/\left( {{h \cdot v \cdot \Delta}\quad f} \right)} + {{NF}_{EDFA}\left( {Pi}_{NEW} \right)}}{{{NF}_{EDFA}\left( {Pi}_{OLD} \right)} \cdot {Gain}}}\end{matrix} & (7)\end{matrix}$

The relation of the equation (7) can be converted to a logarithmicvalue, to be expressed by the following equation (7)′. However, itutilizes the relation Gain=Pi_(NEW)/Pi_(OLD). $\begin{matrix}{{NF}_{{DRA} + {{EDFA}{\lbrack{dB}\rbrack}}} = {{{10 \cdot \log}\left\{ {{P_{ASS}/\left( {{h \cdot v \cdot \Delta}\quad f} \right)} + {{NF}_{EDFA}\left( {Pi}_{NEW} \right)}} \right\}} - {{NF}_{{EDFA}{\lbrack{dB}\rbrack}}\left( {Pi}_{OLD} \right)} - \left( {{Pi}_{{NEW}{\lbrack{dB}\rbrack}} - {Pi}_{{OLD}{\lbrack{dB}\rbrack}}} \right)}} & (7)^{\prime}\end{matrix}$

In the equation (7)′ given above, Pi_(OLD) and NF_(EDFA[dB])(Pi_(OLD))are fixed values. Therefore, to minimize a value of the noise figureNF_(DRA+EDFA[DB]) of the overall apparatus, a value of the followingequation (8) may be minimized. $\begin{matrix}\frac{{P_{ASS}/\left( {{h \cdot v \cdot \Delta}\quad f} \right)} + {{NF}_{EDFA}\left( {Pi}_{NEW} \right)}}{{Pi}_{NEW}} & (8)\end{matrix}$

Accordingly, input light power Pi_(NEW) that makes the value of theequation (8) minimal is obtained by using ASS light power P_(ASS)calculated from the equation (1), and this input light power Pi_(NEW) isset to the input light power target value of the EDFA 20. In this way,the noise figure of the overall apparatus can be minimized.

It is known that the noise figure NF_(EDFA) of the EDFA generally hasdependence on input light power Pi as shown in FIG. 5. In other words,the noise figure NF_(EDFA) of the EDFA has dependence such that itremains substantially constant when input light power Pi of the EDFA isa boundary value Pi_(TH) or less, and increases when input light powerPi exceeds the boundary value Pi_(TH). This input light power dependenceof the noise figure NF_(EDFA) can be numerically formulated as expressedby the following equation (9), for example: $\begin{matrix}\begin{matrix}{{{{When}\quad {Pi}} \leqq {Pi}_{TH}},} & {{{NF}_{EDFA}({Pi})} = F} \\{{{{When}\quad {Pi}} > {Pi}_{TH}},} & {{{NF}_{EDFA}({Pi})} = {b \cdot {Pi}^{a}}} \\{~~} & {{{NF}_{{EDFA}{\lbrack{dB}\rbrack}}({Pi})} = {b + {a \cdot {Pi}_{\lbrack{dB}\rbrack}}}}\end{matrix} & (9)\end{matrix}$

where F, a and b are constants, and when Pi=Pi_(TH),NF_(EDFA)(Pi_(TH))=F=b·(Pi_(TH) ^(a)).

The input light power target value Pi_(NEW) that minimizes the value ofthe equation (8) can be concretely set in the following way, forexample, by taking the relation of the formula (9) into consideration.

When the input light power Pi_(NEW) to the EDFA 20 is sufficiently asmall value (Pi≦Pi_(TH)), NF_(EDFA)(Pi) is constant. Therefore, thenoise figure of the overall apparatus can be minimized by settingPi_(NEW) as the denominator to the greatest possible value in theequation (8). In other words, the optimum condition can be accomplishedwhen the input light power Pi_(NEW) to the EDFA is the greatest.

When the input light power Pi_(NEW) to the EDFA 20 is sufficiently agreat value (Pi>Pi_(TH)), on the other hand, the equation (9) issubstituted for the equation (8) to give the following modified equation(8)′. $\begin{matrix}{\frac{{P_{ASS}/\left( {{h \cdot v \cdot \Delta}\quad f} \right)} + {b \cdot {Pi}_{NEW}^{a}}}{{Pi}_{NEW}} = {\frac{P_{ASS}/\left( {{h \cdot v \cdot \Delta}\quad f} \right)}{{Pi}_{NEW}} + {b \cdot {Pi}_{NEW}^{a - 1}}}} & (8)^{\prime}\end{matrix}$

When a≦1 in the equation (8)′ given above, that is, when an NF slope ofthe EDFA 20 does not exceed 1 dB/dB, the noise figure of the overallapparatus can be reduced to minimum by setting the input light powerPi_(NEW) to the greatest possible value. When a>1, that is, when the NFslope exceeds 1 dB/dB, the noise figure of the overall apparatus can bereduced to minimum by inversely calculating the value of Pi_(NEW) whenthe value of the equation (8)′ described above becomes minimal.

Incidentally, the cases about the equation (8)′ are classified byjudging whether or not the NF slope (value of a) exceeds 1 dB/dB.However, the classification of the cases is not strictly limited to 1dB/dB, but may be set depending on whether or not the noise figure ofthe EDFA can be substantially improved with respect to the increase ininput light power Pi_(NEW).

When the computing section 14 sets the input light power target value ofthe EDFA 20 in this way, the target value is transmitted to thecontrolling section 15. The controlling section 15 adjusts the drivingcondition of the excitation light source 10 so that the input lightpower of the EDFA 20 coincides with the target value from the computingsection 14, thereby controlling automatically the Raman excitation lightpower. Here, the monitoring result of the monitor 32, that measuredactual input light power to the EDFA 20, is transmitted to thecontrolling section 15, and the controlling section 15 executes afeedback control for making the input light power to the EDFA 20reliably constant at the target value. However, when the relation of theinput light power value of the EDFA 20 with respect to the drivingcondition of the excitation light source 10 is clarified in advance,this feedback control may be omitted.

Since a power level of the WDM signal input to the EDFA 20 isautomatically controlled in this way to the target value set by thecomputing section 14 by adjusting the supply condition of Ramanexcitation light, the noise figure of the overall optical amplificationapparatus constituted by combining the DRA and the EDFA becomes minimal.In consequence, the noise characteristics of the optical amplificationapparatus can be optimized through the control made inside its ownapparatus, to realize an optical amplification apparatus utilizing Ramanamplification having excellent noise characteristics. When such anoptical amplification apparatus is employed to construct an opticaltransmission system shown in FIG. 8, the improvement of the transmissioncharacteristics by distributed Raman amplification can be independentlyoptimized and adjusted at each node. This effect is particularlyadvantageous, since it enables to take flexible counter-measure for sucha situation where variance of the loss of transmission path fiber isgreat or where this optical amplification apparatus is installed at arepeating stage inside an optical network through optical ADM or anoptical cross-connect node and its installation environment is likely tofluctuate, for example,.

Incidentally, in the first embodiment described above, ASS light poweris computed by using the monitoring result of Raman excitation light andthe input light power target value to the EDFA 20 is set so that thevalue of the equation (8) finally becomes minimal. However, the methodof setting the input light power target value of the EDFA 20 in thepresent invention is not limited to this method.

For example, when the NF slope of the EDFA is 1 dB/dB or less (when a≦1in the equation (9); a general EDFA is often expected to operate at theNF slope of not greater than 1 dB/dB), the noise figure of the overallapparatus can be reduced to minimum when the input light power Pi_(NEW)of the EDFA 20 is set to the greatest possible value withoutparticularly conducting the calculation as is made in the firstembodiment. This means that a maximum value of input dynamic range ofthe EDFA is set as the input light power target value of the EDFA.However, when the pump power limit is reached before the input lightpower of the EDFA reaches the maximum value of input dynamic range, thatis, when the power of Raman excitation light output from the excitationlight source reaches the maximum value, the input light power of theEDFA corresponding to the Raman excitation light power at that point isset as the target value.

Next, a second embodiment of the present invention will be explained. Inthe second embodiment, consideration will be made on an opticalamplification apparatus suitable for a WDM optical communication systemin which so-called “C band” WDM signal light having a wavelength band of1,550 nm and so-called “L band” WDM signal light having a wavelengthband of 1,580 nm, for example, are collectively transmitted.

FIG. 6 is a block diagram showing the construction of the opticalamplification apparatus according to the second embodiment. Likereference numerals are used in this figure to identify like constituentportions as in the first embodiment shown in FIG. 1.

In FIG. 6, the construction of this optical amplification apparatusdifferent from that of the first embodiment resides in the followingpoints. A first different point is that a plurality (three in thefigure, for example) of excitation light sources 10 ₁ to 10 ₃ havingmutually different wavelengths are provided in this embodiment.Wavelengths λ_(RP1) to λ_(RP3) generated by these excitation lightsources 10 ₁ to 10 ₃ are multiplexed by a WDM coupler 16, an d thensupplied to a transmission path 3 through an optical coupler 12. A partof Raman excitation light having each wavelength λ_(RP1) to λ_(RP3)generated by each excitation light source 10 ₁ to 10 ₃ is branched byeach optical coupler 11 ₁ to 11 ₃ and is monitored by each monitor 13 ₁to 13 ₃. Each monitoring result is sent to a computing controllingsection 40. A sec on d different point pertains to the construction onthe EDFA side. Namely, the EDFA side is constructed corresponding to theC band and the L band , respectively. The constructions other than theconstructions described above are the same as those of the firstembodiment.

More particularly, the construction on the EDFA side includes a WDMcoupler 51 for demultiplexing WDM signal light having passed through theoptical coupler 12 to the C band and the L band, a C band EDFA 20 _(C)for amplifying WDM signal light of the C band demultiplexed by the WDMcoupler 51, an L band EDFA 20 _(L) for amplifying WDM signal light ofthe L band demultiplexed by the WDM coupler 51, and a WDM coupler 52 formultiplexing output light of the C band EDFA 20 _(C) and output light ofthe L band EDFA 20 _(L), to output to the transmission path. Here,optical couplers 31 _(C) and 31 _(L) are interposed between the WDMcoupler 51 and the C band EDFA 20 _(C) and between the WDM coupler 51and the L band EDFA 20 _(L), respectively. Monitors 32 _(C) and 32 _(L)monitor input light power of the C band EDFA 20 _(C) and input lightpower of the L band EDFA 20 _(L), respectively, and each monitoringresult is sent to a computing controlling section 14. Incidentally, thecomputing controlling section 40 is assembled by gathering the computingsection 14 and the controlling section 15 in the first embodiment intoone block, and exhibits the same functions of these computing section 14and controlling section 15.

In the optical amplification apparatus having the construction describedabove, Raman excitation light having three wavelengths λ_(RP1) toλ_(RP3) set in advance so as to correspond to the C band the L band aremultiplexed the WDM coupler 16, and then multiplexed light is suppliedto the transmission path 3 through the optical coupler 12. At this time,a part of Raman excitation light of each wavelength is branched by theoptical coupler 11 ₁ to 11 ₃ and is sent to each monitor 13 ₁ to 13 ₃.Each monitor 13 ₁ to 13 ₃ monitors Raman excitation light power of eachwavelength, and outputs the monitoring result to the computingcontrolling section 40.

The computing controlling section 40 computes total power Ass_(C),Ass_(L) of ASS light of each band by using the Raman excitation lightpower of each wavelength in accordance with the following equations(1_(C)) and (1_(L)). Incidentally, the equations (1_(C)) and (1_(L))represent an example of the relational formulas of when the number i ofthe excitation light sources for Raman amplification is 3 in theafore-mentioned equation (1), the influences of inter-pump Raman aretaken into consideration, and the relation between total power of ASSlight and power of Raman excitation light is approximated by thequadratic function to improve accuracy. $\begin{matrix}\begin{matrix}{{Ass}_{C} = \quad {{c\quad {m_{1} \cdot 10^{\frac{{c\quad {d_{2} \cdot {({{cp}_{1} \cdot {Pu}_{1}})}^{2}}} + {c\quad {d_{1} \cdot {({{{cp}_{1} \cdot {Pu}_{1}} - {d_{12} \cdot {cp}_{1}^{2} \cdot {Pu}_{1}^{2} \cdot {cp}_{2} \cdot {Pu}_{2}} - {d_{31} \cdot {cp}_{3} \cdot {Pu}_{3} \cdot {cp}_{1}^{2} \cdot {Pu}_{1}^{2}}})}}} + {c\quad d_{0}}}{10}}}} +}} \\{\quad {{c\quad {m_{2} \cdot 10^{\frac{{c\quad {d_{2} \cdot {({{cp}_{2} \cdot {Pu}_{2}})}^{2}}} + {c\quad {d_{1} \cdot {({{{cp}_{2} \cdot {Pu}_{2}} - {d_{23} \cdot {cp}_{2}^{2} \cdot {Pu}_{2}^{2} \cdot {cp}_{3} \cdot {Pu}_{3}} + {d_{12} \cdot {cp}_{1} \cdot {Pu}_{1} \cdot {cp}_{2}^{2} \cdot {Pu}_{2}^{2}}})}}} + {c\quad d_{0}}}{10}}}} +}} \\{\quad {c\quad {m_{3} \cdot 10^{\frac{{c\quad {d_{2} \cdot {({{cp}_{3} \cdot {Pu}_{3}})}^{2}}} + {c\quad {d_{1} \cdot {({{{cp}_{3} \cdot {Pu}_{3}} + {d_{31} \cdot {cp}_{3}^{2} \cdot {Pu}_{3}^{2} \cdot {cp}_{1} \cdot {Pu}_{1}} + {d_{23} \cdot {cp}_{2} \cdot {Pu}_{2} \cdot {cp}_{3}^{2} \cdot {Pu}_{3}^{2}}})}}} + {c\quad d_{0}}}{10}}}}}\end{matrix} & \left( 1_{C} \right) \\\begin{matrix}{{Ass}_{L} = \quad {{l\quad {m_{1} \cdot 10^{\frac{{l\quad {d_{2} \cdot {({{lp}_{1} \cdot {Pu}_{1}})}^{2}}} + {l\quad {d_{1} \cdot {({{{lp}_{1} \cdot {Pu}_{1}} - {d_{12} \cdot {lp}_{1}^{2} \cdot {Pu}_{1}^{2} \cdot {lp}_{2} \cdot {Pu}_{2}} - {d_{31} \cdot {lp}_{3} \cdot {Pu}_{3} \cdot {lp}_{1}^{2} \cdot {Pu}_{1}^{2}}})}}} + {l\quad d_{0}}}{10}}}} +}} \\{\quad {{l\quad {m_{2} \cdot 10^{\frac{{l\quad {d_{2} \cdot {({{lp}_{2} \cdot {Pu}_{2}})}^{2}}} + {l\quad {d_{1} \cdot {({{{lp}_{2} \cdot {Pu}_{2}} - {d_{23} \cdot {lp}_{2}^{2} \cdot {Pu}_{2}^{2} \cdot {lp}_{3} \cdot {Pu}_{3}} + {d_{12} \cdot {lp}_{1} \cdot {Pu}_{1} \cdot {lp}_{2}^{2} \cdot {Pu}_{2}^{2}}})}}} + {l\quad d_{0}}}{10}}}} +}} \\{\quad {l\quad {m_{3} \cdot 10^{\frac{{l\quad {d_{2} \cdot {({{lp}_{3} \cdot {Pu}_{3}})}^{2}}} + {l\quad {d_{1} \cdot {({{{lp}_{3} \cdot {Pu}_{3}} + {d_{31} \cdot {lp}_{3}^{2} \cdot {Pu}_{3}^{2} \cdot {lp}_{1} \cdot {Pu}_{1}} + {d_{23} \cdot {lp}_{2} \cdot {Pu}_{2} \cdot {lp}_{3}^{2} \cdot {Pu}_{3}^{2}}})}}} + {l\quad d_{0}}}{10}}}}}\end{matrix} & \left( 1_{L} \right)\end{matrix}$

Where Pu₁ to Pu₃ are Raman excitation light power generated in theexcitation light sources, cm₁ to cm₃ and lm₁ to lm₃ are weightingcoefficients, cd₀ to cd₂ and ld₀ to ld₂ are calculation coefficients,cp₁ to cp₃ and lp₁ to lp₃ are effective pump coefficients, and d₁₂, d₂₃and d₃₁ are inter-pump Raman coefficients, respectively.

After the ASS light total power Ass_(C) and Ass_(L) of the C band andthe L band are calculated, the setting process of input light powertarget value of the EDFA is performed in the same way as in the firstembodiment. Here, the C band EDFA 20 _(C) and the L band EDFA 20 _(L)are connected in parallel to each other as the construction on the EDFAside. Therefore, the input light power target value is set for each bandin such a manner as to correspond to noise characteristics of the C bandEDFA 20 _(C) and noise characteristics of the L band EDFA 20 _(L),respectively.

When the target value corresponding to each band is thus set, thedriving condition of each excitation light source 10 ₁ to 10 ₃ iscontrolled by the computing controlling section 40 so that input lightpower of the C band EDFA 20 _(C) and the L band EDFA 20 _(L) coincidewith the respective target values. Here, a feedback control isperformed, too, by referring to actual input light power for each bandobtained by the monitors 32 _(C), 32 _(L).

Incidentally, the computing controlling section 40 preferably has anadditional function of computing a so-called “SRS tilt” and setting aninitial value of an output light power ratio of each excitation lightsource 10 ₁ to 10 ₃. Computation of the SRS tilt is to roughly compute adifference of gain tilt of WDM signal light that occurs in accordancewith differences in the number of channels and an arrangement (numberand arrangement of optical signals of each wavelength) contained in Cand L band WDM signal light. When the occurrence of the SRS tilt shownat the lower part of FIG. 7A, for example, is computed before Ramanamplification (with excitation light OFF), the initial value of theoutput light power ratio of each excitation light source 10 ₁ to 10 ₃ isset by generating distributed Raman amplification in the transmissionpath so that the wavelength characteristics become flat as shown at theupper part of FIG. 7A. Setting of the initial value in this case isperformed such that the excitation light power ratio of each wavelengthλ_(RP1) to λ_(RP3) providing gain wavelength characteristics shown inFIG. 7B, for example, is obtained to set the initial value. Computationof such an SRS tilt and setting of the initial value of the output lightpower ratio of each excitation light source 10 ₁ to 10 ₃ are conductedat the time of initial activation or restoration from the shutdowncondition of the optical amplification apparatus. Therefore, setting ofthe input light power target value to the EDFA is conducted, aftersetting of the initial value of output power ratio of each excitationlight source 10 ₁ to 10 ₃ has been completed and then WDM signal lighthaving flat wavelength characteristics can be obtained.

According to the second embodiment described above, the noise componentsresulting from Raman amplification can be computed in accordance withthe equations (1_(C)) and (1_(L)) described above by monitoring power ofRaman excitation light having each wavelength, in the construction thatgenerates Raman excitation light by combining a plurality of excitationlight sources 10 ₁ to 10 ₃ having mutually different wavelengths.Therefore, the second embodiment can obtain effects similar to theeffects of the first embodiment. In the construction in which the EDFA20 _(C) and 20 _(L) corresponding to the C and L bands, respectively,are connected in parallel, the input light power target value is set soas to correspond to the noise characteristics of the EDFA 20 _(C), 20_(L) of each band. In this way, the noise figure of the overallapparatus can be reduced to minimum. Furthermore, when the SRS tilt iscomputed to set the initial value of the output power ratio of eachexcitation light source 10 ₁ to 10 ₃, it becomes possible to realize anoptical amplification apparatus having superior amplificationcharacteristics.

In the first and second embodiments described above, a part of Ramanexcitation light output from the front of the excitation light source isbranched by the optical coupler and is then monitored. However, it isalso possible to monitor light output from the back of the excitationlight source (output of back-power PD in the case of LD). When aplurality of excitation light sources are utilized as in the secondembodiment, it is further possible to branch a part of Raman excitationlight multiplexed by the WDM coupler 16, and to demultiplex thisbranched light into each wavelength component by an optical filterhaving a narrow band, thereby monitoring its optical power.

INDUSTRIAL APPLICABILITY

The present invention has large industrial applicability to opticalamplification apparatus used in various optical communication systemsand a controlling method of the optical amplification apparatus, and, inparticular, is effective for the improvement of noise characteristics ofoptical amplification apparatus for amplifying signal light by thecombination with a Raman amplifier and also effective as a controllingtechnology for achieving such an improvement.

What is claimed:
 1. An optical amplification apparatus utilizing Ramanamplification comprising: first optical amplifying means for Ramanamplifying signal light propagated through a Raman amplification mediumby supplying excitation light to said Raman amplification medium; andsecond optical amplifying means for amplifying the signal light outputfrom said first optical amplifying means, wherein said opticalamplification apparatus further comprises: target value setting meansfor setting a target value for minimizing a noise figure of the overalloptical amplification apparatus as to input light power of said secondoptical amplifying means; and excitation light controlling means forcontrolling an excitation light supply condition of said first opticalamplifying means in accordance with the target value set by said targetvalue setting means.
 2. An optical amplification apparatus according toclaim 1, wherein said target value setting means includes an excitationlight power detecting section for detecting excitation light powersupplied to said Raman amplification medium, and a computing section forcomputing noise light power by said first optical amplifying means inaccordance with a detection result of said excitation light powerdetecting section, and setting an input light power target value of saidsecond optical amplifying means for minimizing the noise figure of theoverall optical amplification apparatus on the basis of said computednoise light power and on the basis of noise characteristics of saidsecond optical amplifying means.
 3. An optical amplification apparatusaccording to claim 1, wherein said target value setting means sets amaximum value of input dynamic range of said second optical amplifyingmeans to the input light power target value of said second opticalamplifying means.
 4. An optical amplification apparatus according toclaim 3, wherein when excitation light power of said first opticalamplifying means reaches a maximum value before the input light power ofsaid second optical amplifying means reaches the maximum value of saidinput dynamic range, said target value setting means sets the inputlight power target value of said second optical amplifying means so asto correspond to the maximum value of excitation light power of saidfirst optical amplifying means.
 5. An optical amplification apparatusaccording to claim 1, further comprising; input light power detectingmeans for detecting the input light power of said second opticalamplifying means, wherein said excitation light controlling meanscontrols the excitation light supply condition of said first opticalamplifying means so that a detection result of said input light powerdetecting means coincides with the target value set by said target valuesetting means.
 6. An optical amplification apparatus according to claim1, wherein when said second optical amplifying means includes aplurality of optical amplifying sections connected in parallel with oneanother, said excitation light controlling means sets an input lightpower target value corresponding to each of said optical amplifyingsections.
 7. An optical amplification apparatus according to claim 1,wherein said second optical amplifying means includes an optical fiberamplifier using a fiber doped with a rare earth element.
 8. An opticalamplification method comprising: a first optical amplifying process ofRaman amplifying signal light propagated through a Raman amplificationmedium by supplying excitation light to said Raman amplification medium;and a second optical amplifying process of amplifying the signal lightoutput from said first optical amplifying process, wherein a targetvalue for minimizing a noise figure of the overall optical amplificationmethod as to input light power of said second optical amplifying processis set and an excitation light supply condition of said first opticalamplifying process is controlled in accordance with said target value.9. An optical amplification apparatus comprising: a first opticalamplifier Raman amplifying signal light propagated through a Ramanamplification medium by supplying excitation light to said Ramanamplification medium; a second optical amplifier amplifying the signallight output from said first optical amplifier; a target value settingunit setting a target value for minimizing a noise figure of the overalloptical amplification apparatus as to input light power of said secondoptical amplifier; and an excitation light controller controlling anexcitation light supply condition of said first optical amplifier inaccordance with the target value set by said target value setting unit.10. An optical amplification apparatus according to claim 9, whereinsaid target value setting unit includes an excitation light powerdetecting section detecting excitation light power supplied to saidRaman amplification medium, and a computing section computing noiselight power by said first optical amplifier in accordance with adetection result of said excitation light power detecting section, andsetting an input light power target value of said second opticalamplifier for minimizing the noise figure of the overall opticalamplification apparatus on the basis of said computed noise light powerand on the basis of noise characteristics of said second opticalamplifier.
 11. An optical amplification apparatus according to claim 9,wherein said target value setting unit sets a maximum value of inputdynamic range of said second optical amplifier to the input light powertarget value of said second optical amplifier.
 12. An opticalamplification apparatus according to claim 11, wherein when excitationlight power of said first optical amplifier reaches a maximum valuebefore the input light power of said second optical amplifier reachesthe maximum value of said input dynamic range, said target value settingunit sets the input light power target value of said second opticalamplifier so as to correspond to the maximum value of excitation lightpower of said first optical amplifier.
 13. An optical amplificationapparatus according to claim 9, further comprising; an input light powerdetector detecting the input light power of said second opticalamplifier, wherein said excitation light controller controls theexcitation light supply condition of said first optical amplifier sothat a detection result of said input light power detector coincideswith the target value set by said target value setting unit.
 14. Anoptical amplification apparatus according to claim 9, wherein when saidsecond optical amplifier includes a plurality of optical amplifyingsections connected in parallel with one another, said excitation lightcontroller sets an input light power target value corresponding to eachof said optical amplifying sections.
 15. An optical amplificationapparatus according to claim 9, wherein said second optical amplifierincludes an optical fiber amplifier using a fiber doped with a rareearth element.
 16. An apparatus comprising: a first optical amplifiercomprising a Raman amplification medium supplied with excitation lightso that a signal light is amplified by Raman amplification as the signallight travels through the Raman amplification medium, to thereby outputa Raman amplified signal light from the first optical amplifier; asecond optical amplifier receiving the Raman amplified signal lightoutput from the first optical amplifier, and amplifying the receivedRaman amplified signal light; a target value setting unit setting atarget value of a power of the Raman amplified signal light as receivedby the second optical amplifier to minimize a total noise figure of thefirst and second optical amplifiers taken together; and a controllercontrolling the excitation light supplied to the Raman amplificationmedium of the first optical amplifier in accordance with the targetvalue set by the target value setting unit.
 17. An apparatus accordingto claim 16, wherein the target value setting unit includes a detectordetecting excitation light power of the excitation light supplied to theRaman amplification medium, and a computing section computing noiselight power by the first optical amplifier in accordance with adetection result to the detector, and setting the target value on thebasis of said computed noise light power and on the basis of noisecharacteristics of the second optical amplifier.
 18. An apparatusaccording to claim 16, wherein the target value setting unit sets amaximum value of input dynamic range of the second optical amplifier tothe target value.
 19. An apparatus according to claim 18, wherein whenexcitation light power of the excitation light supplied to the Ramanamplification medium reaches a maximum value before the power of theRaman amplified signal light as received by the second optical amplifierreaches the maximum value of the input dynamic range, the target valuesetting unit sets the target value so as to correspond to the maximumvalue of excitation light power of the excitation light supplied to theRaman amplification medium.
 20. An apparatus according to claim 16,further comprising: a detector detecting the power of the Ramanamplified signal light as received by the second optical amplifier,wherein the controller controls the excitation light supplied to theRaman amplification medium so that a detection result of the detectorcoincides with the target value set by said target value setting unit.21. An apparatus according to claim 16, wherein the second opticalamplifier includes a plurality of optical amplifying sections connectedin parallel with one another, and the controller sets a target valuecorresponding to each of the optical amplifying sections.
 22. Anapparatus according to claim 16, wherein the second optical amplifier isan optical fiber amplifier using a fiber doped with a rare earthelement.
 23. An apparatus comprising: a first optical amplifiercomprising a Raman amplification medium supplied with excitation lightso that a signal light is amplified by Raman amplification as the signallight travels through the Raman amplification medium, to thereby outputa Raman amplified signal light from the first optical amplifier; asecond optical amplifier receiving the Raman amplified signal lightoutput from the first optical amplifier, and amplifying the receivedRaman amplified signal light; and a controller controlling theexcitation light supplied to the Raman amplification medium of the firstoptical amplifier so that the Raman amplified signal light as receivedby the second optical amplifier is at a power level which causes a totalnoise figure of the first and second optical amplifiers, taken together,to be minimized.
 24. An apparatus comprising: a Raman amplificationmedium supplied with excitation light so that a signal light amplifiedby Raman amplification as the signal light travels through the Ramanamplification medium, to thereby output a Raman amplified signal lightfrom the Raman amplification medium; an optical amplifier receiving theRaman amplified signal light output from the Raman amplification medium,and amplifying the received Raman amplified signal light; and acontroller controlling the excitation light supplied to the Ramanamplification medium so that the Raman amplified signal light asreceived by said optical amplifier is at a power level which causes atotal noise figure due to amplification in the Raman amplification andsaid optical amplifier, taken together, to be minimized.
 25. Anapparatus comprising: a first optical amplifier comprising a Ramanamplification medium supplied with excitation light so that a signallight is amplified by Raman amplification as the signal light travelsthrough the Raman amplification medium, to thereby output a Ramanamplified signal light from the first optical amplifier; a secondoptical amplifier receiving the Raman amplified signal light output fromthe first optical amplifier, and amplifying the received Raman amplifiedsignal light; and means for controlling the excitation light supplied tothe Raman amplification medium of the first optical amplifier so thatthe Raman amplified signal light as received by the second opticalamplifier is at a power level which causes a total noise figure of thefirst and second optical amplifiers, taken together, to be minimized.